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Tyrannosaurus rex is part of the carnivorous groups of dinosaurs that, according to new research, maintained a stable level of biodiversity leading up to the mass extinction at the end of the Cretaceous.

"I think our study highlights the fact that we still have a long way to go until we fully understand the extinction of the dinosaurs," lead author Stephen Brusatte, a Columbia University graduate student affiliated with the American Museum of Natural History's Division of Paleontology, told Discovery News. The study was published in the latest issue of Nature Communications.

"There are a couple of things we know for sure," he added. "We know a large asteroid or comet hit the planet about 65.5 million years ago, right when the dinosaurs completely disappeared from the fossil record."

"We also know there was massive volcanism and major sea level changes at this time. We now also know that at least some groups of dinosaurs were undergoing long-term declines in biodiversity during the final 12 million years of the Cretaceous, at least in North America."

The study presents the first look at dinosaur extinction based on morphological disparity, meaning the variability of body structure within particular groups of dinosaurs. The more the variability in a species, generally, the healthier the population was.

Earlier research was based almost always on estimates of change in the number of dinosaur species over time — but that can be affected by uneven sampling within the fossil record.

Some geological formations, for example, tend to preserve dinosaur remains better than others.

Brusatte and his team calculated morphological disparity for seven major dinosaur groups using databases that include wide-ranging characteristics about the intricate skeletal structure of nearly 150 different species.

They discovered that large-bodied, bulk-feeding plant-eaters were dying out long before the natural disasters of 65.5 million years ago. These animals included hadrosaurs and ceratopsids.

On the other hand, small plant-eaters (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs) and huge plant-eaters without advanced chewing abilities (sauropods) remained fairly stable over the same period of time.

While the die-off of the larger species remains a mystery, Brusatte said, "Something was going on with large herbivores in the late Cretaceous, at least in North America. Maybe it was the fact that the local environments were in flux due to drastic sea level changes and mountain building at the time."

He explained that plant-eaters may have felt the effects of a changing land area first since they sat at the bottom of the food chain.

"Given a few more million years we would have seen declines in other dinosaur groups higher up in the food chain," he said.

Paul Upchurch, a University College London paleobiologist doesn't buy it and stands by the idea that a big asteroid wiped out the dinosaurs.

"First, only some dinosaur groups show reduced disparity in the final 12 million years, while other groups continue to do well. So this study could actually be taken as evidence in favor of a sudden extinction," Upchurch added. "We need a mechanism that explains why the smaller dinosaurs and large sauropods died out suddenly at the end of the Cretaceous."

Second, he argues that a more extensive look at all dinosaur history is needed to see if such population declines happened more than once over the 165 million years that dinosaurs were in existence.

"The decline in disparity during the final 12 million years might merely be 'evolutionary business as usual' and have little to do with the true final extinction," he said.

Brusatte agrees that his team's findings are debatable, "but at the very least we can't envision the latest Cretaceous as a static, idyllic lost world that was suddenly exterminated by an asteroid impact.

"Instead, the dinosaurs living during this time were undergoing major changes before the asteroid hit."

Yeah, it's cliche to say Tyrannosaurus rex was deadly. But the tyrant king was likely true to the billing. Its bone-crushing jaws could splinter prey like toothpicks, after all. And the beast was big, up to 40 feet long, 20 feet tall, and may have topped the scales at nearly 16,000 pounds.

The king walked on two legs over a vast territory in western North America at the end of the Cretaceous, about 65 million years ago. Scientists wonder whether T. rex was more a lumbering scavenger or a quick and agile predator, but dead or alive, its meals were big, meaty and bloody.

T. rex wasn't the only deadly dino, however. The globe was filled vicious killers. Click the arrows above to see more.

— John Roach, msnbc.com contributor

Family diner

Demetrios M. Vital

Family meals for Majungasaurus crenatissimus were often bloody affairs of the grimmest sort: Kin were the main course. The evidence of its cannibalistic ways comes from telltale tooth marks on Majungasaurus bones that match up perfectly with the size and spacing of teeth in its jaws.

The 20-foot-long dinosaur stalked the plains of Madagascar about 70 million years ago. At that time, the crime-scene investigators said, pickings were often slim. Their data indicate Majungasaurus fed on dried out "dino jerky" from its plant-eating compatriots as well as members of its own species. Cannibalism was likely just as common among dinosaurs as it is among living animals, but the evidence is rare.

Eocarcharia dinops' brow was swollen into a massive band of bone. The menacing head piece may have been used as a battering ram against rivals and to attract potential mates. Its blade-shape teeth were reserved for disabling live prey and severing their body parts.

The 110-million-year-old beast and its snout-nosed, gut-and-carcass-scavenging relative Kryptops palaios were discovered in Africa's Sahara Desert. The approximately 7-foot-tall and 25-foot-long duo likely teamed up with a third carnivore and feasted on the long-necked plant-eater Nigersaurus.

Vicious rocker

Parsons

The toothy Masiakasaurus knopfleri likely speared prey with its forward projecting front teeth and then sliced and tore the captives into chewy chunks with its bladelike rears. This type of tooth arrangement is otherwise unknown in predatory dinosaurs.

Scientists believe the German Shepherd sized beast feasted on fish, lizards, and other critters on the southern supercontinent Gondwana in the Late Cretaceous period, about 65 to 70 million years ago.

The first part of this dinosaur's name means "vicious lizard" and the second part is derived from the Dire Straits singer and guitarist Mark Knopfler, whose music inspired the discoverers as they toiled on the African island nation of Madagascar.

Biggest carnivore?

Prof. Rodolfo Coria / Ap

At about 40 feet long and weighing an estimated 6 tons, Mapusaurus roseae was one of the biggest meat-eating dinosaurs to ever stalk the Earth. Bones of several individuals were discovered in one place, suggesting the giants may have hunted in packs that could have toppled perhaps the largest dinosaur that ever lived — a 100-foot-long plant eater called Argentinosaurus.

Mapusaurus lived about 100 million years ago and was bigger than well known Tyrannosaurus rex and possibly larger than its older cousin, Gigantosaurus. Its teeth were narrow and blade like, made for slicing its prey. The bones were discovered in the Patagonia region of Argentina.

Falcarius utahensis is the living image of a vicious meat eater trying to go vegetarian. Scientists aren't sure if the bird-like relative of Velociraptor had fully kicked its taste for flesh, but its meat-cutting teeth had shrunk to leaf-cutting size and its gut had expanded sufficiently to ferment plants.

Falcarius walked on two legs and stood about 4.5 feet tall. Head to tail, it was about 13 feet long and wielded sharp, curved, four-inch long claws — perhaps to grab a bite when it fell off the wagon? The dinosaur lived during the Early Cretaceous, about 125 million years ago, in what is today Utah.

Plant ripper

utah.edu

No plants were safe from Gryposaurus monumentensis, a big boned, duck-billed dinosaur that could have eaten any vegetation it stumbled across. Its massive skull packed more than 300 teeth for slicing up fibrous greens. Hundreds more replacement teeth rested in its jawbone for the call-up to action.

The dinosaur lived in the Late Cretaceous, about 75 million years ago on the western side of a giant, shallow ocean that split North America at that time. The plant-munching beast may have reached 30 feet long as an adult and had a 3-foot-long head.